The present invention pertains to the field of personnel locating systems. Systems of this general type have been proposed in the prior art for use in buildings where it is necessary to keep track of the movements of certain key personnel from one room or area to another. One example is in a hospital or clinic, where doctors move from one room to another, and there is a need to know the location of each doctor at all times in case they are needed in emergencies. Personnel locator systems are useful in other situations also, so that key or supervisory people in an office, school, plant or the like can be located on short notice.
Personnel locator systems generally comprise small electronic transmitters or transceivers to be worn by the individuals, and a network of receivers or transceivers installed in various rooms or areas to be visited by the personnel. Signals are generally sent to a central location and processed so that the location of the individual transmitters can be displayed or indicated.
While prior art locating systems have achieved their basic objective of tracking movements of the personnel, prior art systems have suffered disadvantages of complexity and high cost, factors which have been serious obstacles to adoption of such systems in places where they would otherwise be useful. One type of prior art personnel locator makes use of ultrasonic transmitters to be worn by key personnel, with each transmitter operating on a different carrier frequency. This requires that each room has multiple receivers, one for each frequency, or in the alternative complex receivers tuned to multiple frequencies. Since a different frequency is required for each person to be monitored, it is very evident that as the number of key personnel to be monitored increases, the cost of the total system increases at a disproportionately high rate, since an additional receiver for each additional frequency must be provided in each room. This type of prior art system is shown in U.S. Pat. No. 3,439,320 to Ward.
Another prior art system is shown in U.S. Pat. No. 3,696,384 to Lester. In this ultrasonic system coded pulses are transmitted in all rooms, and the portable unit having the particular code will respond by transmitting a reply signal which is picked up and used for displaying the location of the particular unit. Although this system requires only a single transmission frequency, it requires both transmitters and receivers or transceivers, in each room and for each portable unit. U.S. Pat. No. 3,739,329 to Lester uses a combination of ultrasonic and radio frequency transmission with particular frequencies or codes for the portable units, but the system also requires transceivers at the portable units, the central console, and a plurality of remote stations.
In addition to the complexity and cost represented by the multiple receivers or transmitter-receivers in the prior art systems as discussed above, such systems may be subject to a possible further disadvantage in that erroneous locations will be displayed if one or more of the key personnel inadvertently carry the wrong portable unit. This could happen, for example, if the portable units are all checked into a central area for battery recharging at the end of the day, and checked out again for use in the morning. Since each portable unit is wired or electronically adjusted for a discrete code or frequency, a person who inadvertently takes the wrong unit will be transmitting erroneous signals as to someone else's location. Of course it is not possible to eliminate entirely the possibility of such inadvertent errors by system design, but one aspect of the present invention provides a technique for minimizing the possibility of such errors.